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02-14-2021, 04:01 PM   #1
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Image circle?

Hi Guys,

Can someone tell me if the image circle of the 645 DFA lenses is large enough to use on a 645nii film camera?

Thanks
Butch

02-14-2021, 05:09 PM - 1 Like   #2
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Yes, absolutely. That's the difference between DA and DFA designations. 645 DA lenses are sold for cameras with crop medium format sensors 33mm x 44mm, i.e. 645D and 645Z (some claim that there may be acceptable coverage at some focal lengths on full frame, but it's not what they're designed to do). The 645 DFA lenses cover full frame 645 by design....

Last edited by Ed Hurst; 02-15-2021 at 03:26 PM.
02-15-2021, 04:34 PM   #3
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QuoteOriginally posted by Ed Hurst Quote
Yes, absolutely. That's the difference between DA and DFA designations. 645 DA lenses are sold for cameras with crop medium format sensors 33mm x 44mm, i.e. 645D and 645Z (some claim that there may be acceptable coverage at some focal lengths on full frame, but it's not what they're designed to do). The 645 DFA lenses cover full frame 645 by design....
Thanks Ed,

I was looking at a 645 DFA 25 to use on a 645 Nii. I wasnt sure if the DFA would cover the entire film frame area.
02-15-2021, 04:55 PM   #4
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It will! I have no first-hand knowledge of the electrical functioning of this new era lens on the 645 Nii - the lens has no mechanical aperture ring - which may affect the modes and methods you can use with it. Others may be able to provide more insight there. But the 25 DFA will certainly cover the entire film area. The 25 DA (with its longer hood and different internal baffles) will, I think, vignette. So important that you aim for the DFA.

02-15-2021, 04:55 PM   #5
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Yes, the DFA does. It was quickly replaced by the DA 25mm, which is optically identical but has an extended non-removable hood that vignettes on full frame 645. Unfortunately, both are now discontinued.
02-15-2021, 05:13 PM   #6
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QuoteOriginally posted by Butch5 Quote
Thanks Ed,

I was looking at a 645 DFA 25 to use on a 645 Nii. I wasnt sure if the DFA would cover the entire film frame area.
Terminology is such a point of confusion in that so-called 645 FF digital cameras on the current market actually use a crop sensor. Such is not true with Pentax since the brand has never made a camera for the new format. The lenses for all of the current line up support either the 645 digital cameras (DA lenses) or both the digital and film 645 cameras (D FA and FA lenses). The FA lenses are fully compatible with your camera and the D FA line are backwards compatible to the extent of support for shared features such as in-lens focus motors.


Steve
02-15-2021, 05:31 PM - 1 Like   #7
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It is [or was] possible to find, by searching, a web page at Pentax that shows a refurbished 25 DFA, but there is [or was] no button action to buy it. The implication is that Pentax occasionally get lens assemblies for refurbishment that they sell. Otherwise, lying in wait on eBay seems the next most probable acquisition path.

I will have to remember to test whether I can get my DFA to change aperture size on my 645N. I expect that it will because it has the usual aperture lever at the flange plane. I don't recall the N having a camera control for remotely setting the aperture, other than automatically in relevant programmed modes. In other words, Ap mode might only work full open. A quick scan of the NII manual doesn't show an obvious way of changing aperture remotely. If there is such, feel free to enlighten me.

On further thought, I think that the lens is in A mode, such that the camera will control it so long as the camera is set in A mode. I'm not sure what happens if the lens is in A mode and the exposure is set to a time value and the ISO is fixed, hopefully the aperture is adjusted appropriately. TBD

Last edited by kaseki; 02-15-2021 at 05:40 PM.
02-15-2021, 06:57 PM   #8
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Thanks, Kaseki - that nicely summarises the usability questions I was aware of (but couldn't remember in enough detail to explain!). The lens certainly covers full frame 645 film. But what options you have available for controlling the aperture with the body - that's the key question. Looking forward to knowing that definitely!

And, re. Paul the Sunman's message above, the hood is not the only difference between the 25mm DFA and the 25mm DA, but the upshot for this thread is that the DFA covers full frame 645 and the DA doesn't.

02-15-2021, 10:12 PM   #9
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My guess, pending testing, is that given an ISO setting, the aperture will follow the AE programming (+/- exposure offset) as a function of scene light and the chosen shutter timing. So a static scene that has a particular need for a specific aperture for, say, depth of field, could get that aperture (approximately) by changing shutter speed until the view-finder displayed f/no was close to the desired f/no.

Note to self: Taking notes is advised to compare with the data on the film edge when developed.

Now to think of an appropriate scene to blow a roll of film, and environmental conditions I can stand. I think for easiest camera auto exposure estimation, I need to wait for the snow to melt.
02-15-2021, 10:50 PM   #10
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So how would one manually shoot a super-long exposure, using Bulb, and be sure of using a given aperture? The sort of thing one might do a lot of if using an ND on a landscape shot, obviously! Or maybe an astro shot.

Last edited by Ed Hurst; 02-15-2021 at 10:58 PM.
02-16-2021, 07:44 AM - 1 Like   #11
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QuoteOriginally posted by Ed Hurst Quote
So how would one manually shoot a super-long exposure, using Bulb, and be sure of using a given aperture? The sort of thing one might do a lot of if using an ND on a landscape shot, obviously! Or maybe an astro shot.
At present, I don't know of a method for direct aperture control such as one might want for B mode. I don't know enough about non-reciprocity of available 120 film stock to guess how well astronomical photography might work. If the aperture stays full open on B, then at least one has some compatibility with generally low ISO value film. Depth of field for star fields and distant terrain might be OK full open. Still, 25-mm no-aperture-ring WR DFA compatibility with FA based cameras may not be as complete as one might wish.

While I can imagine using long exposures on water to fuzz up water motion, I am completely unfamiliar with the principles and purposes of Bulb vs. ND terrain photography so I have no answer there. I guess a slightly longer 645 or 67 lens that has an aperture ring will have to do.

While film vs. focal plane array in portraiture might be a subject for debate, I am reasonably certain that our Z cameras are better for star trails than their film predecessors. Think of all the negative development and scanning needed to make a zillion images for summation (that I think you have reported). And every 12 shots the camera gets disturbed by a film change. And the cost of the film needed. Yikes!
02-16-2021, 03:33 PM   #12
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Thanks, mate. I fully agree with what you say about digital being vastly superior to film for star trail photography. That was more a hypothetical example aimed at finding out if it's possible to control the aperture in such modes, without relying on autoexposure...

It's unlikely to be relevant to me personally, since my film days are largely behind me. More a point of interest, aimed at uncovering issues/work-arounds that may be relevant to people like Butch5 :-). Certainly, shooting manual or bulb with ND or big stopper type filters is not an unlikely application for such a lens on a film body methinks...
02-16-2021, 04:35 PM   #13
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QuoteOriginally posted by Ed Hurst Quote
So how would one manually shoot a super-long exposure, using Bulb, and be sure of using a given aperture?
Near as I can tell, you would not be able to. The rules are similar to that of using D FA K-mount lenses on K-mount film cameras that work similar to the 645NII;* one is limited to P and Tv modes unless willing to shoot wide open.


Steve

(...only has experience with the 645N...lovely camera...)

* The Super Program is a good example. I have used mine in that manner, but doing so is very limiting.
02-16-2021, 04:38 PM   #14
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Thanks for that - it's what I feared was the case...

Butch5 - hope this discussion is helpful to you in deciding on what to do. As you can see, the practical questions are much wider (no pun intended!) than whether the 645DFA lens optically covers a full 645 film frame (which it does)...

Last edited by Ed Hurst; 02-16-2021 at 08:20 PM.
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